This invention relates to battery-powered wireless telephone equipment, such as is used in automobiles or any other wireless telephone equipment which may be subject to a drop in power supply voltage.
As shown in FIG. 1, a wireless telephone system includes wireless telephone apparatus 1,1 communicating by radio signals with base station 2 which is connected by landline to the telephone exchange 3. This arrangement enables calls to be placed between two wireless telephones 1,1 or a wireless telephone 1 and an ordinary telephone 4. The frequencies used in communicating between wireless telephones 1 and the base station 2 comprise a plurality of channels, at least one of which is allocated to control, the remainder being allocated to operations, that is, telephone conversations. A conversation on a vacant operating channel is first established by initiating communication on the control channel and exchanging control information, then switching to the operation channel.
FIG. 2 in a block diagram of conventional wireless telephone equipment 1. (In FIGS. 2, 4, 5, 7, 8 and 10, heavy lines indicate conductors which carry signals; while power-carrying conductors are drawn in medium weight lines.) This equipment may be divided into a transceiver section 21, a control section 22, a telephone instrument section 23, a power supply section 24, and a power source 25, such as the battery in an automobile. Forming a part of transceiver section 21 are transmitter 211, receiver 212, frequency synthesizer 213, filter 214, and antenna 215. Transmitter 211 accepts either the control signal from control section 22 or the audio signal from telephone instrument section 23 and uses the accepted signal to modulate the radio frequency which will be transmitted to base station 2. The modulated radio frequency from transmitter 211 is then passed to antenna 215, to be radiated, via filter 214. Receiver 212 intercepts an incoming modulated radio frequency signal via antenna 215 and filter 214 and demodulates it to either a control signal, which is sent to control section 22, or an audio signal, which is sent to telephone instrument section 23. Frequency synthesizer 213 serves as the local oscillator, supplying radio frequency signals to both transmitter 211 and receiver 212. Filter 214 actually comprises two band pass filters, one for the outgoing signal and one for the incoming signal.
Control section 22 comprises CPU (Central Processing Unit) 221 and a read-write memory, RAM (Random Access Memory) 222. CPU 221 controls the operation of both transceiver section 21 and telephone instrument section 23 in accordance with control data. RAM 222 records the control data required to establish and maintain the conversation, such as the location of the wireless telephone station.
Telephone instrument section 23 comprises a conventional talking circuit 231 to generate outgoing telephone calls and receive incoming telephone calls, and main power switch 232.
Before initiation of a call, CPU 221 sets the output frequencies of frequency synthesizer 213 to the frequencies corresponding to the control channel; location information transmitted by base station 2 is received by receiver 212 and stored in RAM 222.
If the operator of this wireless telephone station initiates a call, the call origination signal is transmitted by transmitter 211 to base station 2 and, according to the command transmitted from the base station, CPU 221 sets the output frequencies of synthesizer 213 to those corresponding to a vacant operating channel. Thereafter, talking circuit 231 is connected to transmitter 211 and receiver 212, and a conversation is established.
If this wireless telephone station is called by another, base station 2 transmits, over the control channel, the call receipt signal along with the appropriate frequency signal. Upon receipt of this information, CPU 221 sets the output frequencies of synthesizer 213 to correspond to the frequencies of the assigned operating channel. Talking circuit 231 is then connected to transmitter 211 and receiver 212 and the conversation is established.
Power supply section 24 comprises voltage monitor 241, power supply control 242, and switching circuit 243. Voltage monitor 241, which monitors the output voltage level of power source 25, normally outputs the active-high logic signal S to power supply control 242 when the monitored voltage exceeds the minimum level necessary to supply any of the sections of the wireless telephone apparatus. If the monitored voltage drop below this minimum level, volage monitor 241 outputs the active-low signal S, that is, the alarm signal, and power supply control 242 disconnects power from all sections of the apparatus. In this way, erroneous operation of any section, due to the supply voltage being too low, is prevented. Power supply control 242 includes relay circuit 244 which operates relay 245, at which point power is interrupted. Switching circuit 243 comrises a switching transistor 246 which is turned on or off by main power switch 232.
In conventional wireless telephone apparatus, such as described above, the minimum voltage level which causes voltage monitor 241 to output an alarm signal is set at the lowest level which will guarantee normal operation of all sections of the apparatus. This results in the limit voltage being set rather high, for example, 10 volts in the case of equipment designed to operate from a 12.6 volt automobile battery. The reason for the narrow tolerance in supply voltage is the inability of frequency synthesizer 213 to properly operate over a large voltage range.
As shown in FIG. 3, frequency synthesizer 213 includes a phase-locked loop (PLL) in which the output frequency is determined by a voltage-controlled oscillator (VCO) 31. The frequency of the output signal f.sub.v of VCO 31 is divided, by a frequency divider 32, by a factor of N. The phase of this signal f.sub.v /N is then compared in phase comparator 34 with the phase of reference frequency f.sub.r of reference oscillator 33. The output voltage of phase comparator 34, which is proportional to the phase difference of its two input signals, is supplied to VCO 31 via filter 35. In this way, the output signal of VCO 31, which is also the output signal of synthesizer 213, is controlled to be Nf.sub.r. By changing the value of N, the divisor of the frequency divider, frequency f.sub.v is changed. If, however, the voltage supplied to either VCO 31 or phase comparator 34 drops, its bias conditions are changed and the output frequency of the VCO will vary momentarily. In an extreme case, lockout of the PLL will occur, causing radiation of signals at an undesired frequency.
As is well known, the output voltage of an automobile battery may be temporarily reduced under certain conditions such as idling with a heavy electrical load or cold starting. (See Society of Automotive Engineers, Recommended Environmental Practices for Electronic Equipment Design (SAE J1211, S4.10.1.) In an automobile containing a wireless telephone, therefore, if a conversation is being maintained during engine starting, and if the start causes battery voltage to drop below the limit voltage, the power supply to all circuits (except RAM 222, which is supplied by a backup battery) will be interrupted, and all circuits will be reinitialized after the start, when battery voltage rises above the limit level. Unfortunately, the conversation is cut off, and reinitialization does not allow it to be resumed. In order to resume the conversation, another call must be originated. This is an undesirable feature of mobile telephone service.
Another undesirable effect of the termination of conversations due to power supply voltage drops is that origination of the call must be done, as it always is, on the single control channel. Consequently, the traffic level on the control channel is increased, and the operating channels are used inefficiently.
A further source of inconvenience due to a supply voltage drop relates to the storage of location information in RAM 222. In wireless telephone systems, a service zone is divided into a plurality of control zones, each with its own base station 2. The base station in each zone periodically transmits location information on a dedicated channel to all wireless telephone stations in the zone notifying the wireless stations that they are within a particular zone. This location information is written by each wireless telephone station into its RAM 222. Whenever the location information changes, for example, when the wireless telephone station moves into a different control zone, the wireless telephone station transmits its identification to the base station for that control zone, so that the base station is always kept informed of which wireless telephone stations are within its control zone. Transmission of this identification from wireless telephone stations 1 to the base stations 2 is done on the control channel.
Despite the existence of backup power to RAM 222, it is possible for an interruption in power to occur, when the voltage level of the wireless telephone's power source 25 drops below the limit level, during the switchover from main to backup power. During this power interruption, CPU 221 may run away and change or delete the control information stored in RAM 222. When power is restored, and location information is again written into RAM 222, it will appear as though the location information has changed, and the control channel will again be used to inform the base station of the presence of the wireless telephone station.
With increased use of the control channel, the probability increases that the channel will be busy and that a call cannot be completed. Service degradation is the result.